Downhole tractors are often employed to drive a downhole tool through a horizontal or highly deviated well at an oilfield. In this manner, the tool may be positioned at a well location of interest in spite of the non-vertical nature of such wells. Different configurations of downhole tractors may be employed for use in such a well. For example, a reciprocating tractor may be utilized which employs separate adjacent sondes with actuatable anchors for interchangeably engaging the well wall. That is, the sondes may be alternatingly immobilized with the anchors against the well wall and advanced in an inchworm-like fashion through the well.
During tractoring operations, the above-noted tractor may proceed downhole, along with several thousand pounds of equipment. The tractor and equipment may be driven thousands of feet into the well for performance of a downhole operation such as the logging operation described below. Regardless, in order to achieve sufficient tractor and equipment advancement, forces are imparted from the tractor toward the well wall through the noted anchors and/or traction elements. In theory, the tractor may thus avoid slippage and achieve the noted advancement through the well.
Often times, tractors are employed in open-hole or uncased wells. For example, a logging application to determine well characteristics may be run in advance of well casing. In this manner, a more direct evaluation of well conditions may be obtained in advance of casing or other downhole fixture placements. Unfortunately, advancement of the tractor through an open-hole well may face particular challenges. More specifically, in open-hole tractoring, the well is defined by the exposed formation alone. Thus, the well is likely to be variable in terms of wall surface characteristics, well diameter, etc. As described further below, the variability in wall surface characteristics in particular, may have a substantially adverse impact on the frictional footing each anchor is able to maintain as it engages the wall for advancement.
For an open-hole well, the degree of hardness or softness of the well wall may vary significantly over the span of several thousand feet of well. As a result, conventional tractoring may be rendered impractical in certain regions of the well due to the degree of softness found at the wall of exposed formation. In order to address this issue, an excess of expansion forces are often applied through the tractor's anchor arms. For example, with the load pulled in mind, forces that should be sufficient for maintaining a frictional footing at the interface of the anchor arms and softer well wall sections are employed to drive the tractor through the well.
Unfortunately, the use of excess expansion forces alone may lead to a host of other problems in terms of achieving proper tractor advancement. For example, in smaller diameter well sections of sufficient hardness, the tractor may simply end up damaging itself via attempts to unnecessarily over expand its anchor arms. Ultimately, this may lead to mechanical failure of the tractor as a result of over-stressed anchor arms. Furthermore, as described below, excess expansion forces may fail to even maintain frictional footing in the first place. That is, even with an unlimited supply of radial force available, softer portions of the well wall may deteriorate as a result of the axial load imparted on the tractor.
As indicated above, even with an unlimited amount of radial force available, frictional footing may be a challenge to maintain due to the axial load that is pulled by the tractor. For example, in order to maintain the inertia of the downhole advancing tractor, wireline cable and other equipment, the tractor generally proceeds downhole in a continuous manner. In other words, the axial load of the entire system is continuously being pulled downhole. As a result, the anchor arms are interchangeably acting not only to radially grab a foothold on the well wall, but also to simultaneously withstand and maintain the downhole pull of an ever increasing load of the system. Thus, there may be periods where a given anchor is responsible for maintaining the axial load of the entire system in spite of incomplete build up radial expansion forces through its arms. When this occurs at softer sections of the open-hole well, the wall surface is prone to crumble and the anchor may fail to maintain a frictional foothold, ultimately resulting in tractor slipping and ceasing of tractor operations.